KR100285648B1 - Heating method of high temperature resistant 310s stainless steel slab - Google Patents

Abstract

PURPOSE: To improve the surface quality of a hot rolled 310S stainless steel slab by lowering excess oxygen portion in heating furnace. CONSTITUTION: In the heating method of austenitic 310S stainless steel slab comprising C 0.08wt.% or less, Cr 24-26wt.%, Ni 19-22wt.%, Mn 2wt.% or less, Si 1.5wt.% or less, B 0.01wt.% or less, a balance of Fe and other inevitable impurities for hot rolling by using COG or BFG, the present invention is characterized in that volume rate of excess oxygen in heating furnace is controlled to be less than 1.5%.

Description

Heating method of heat austenitic stainless steel slab of 310S

The present invention relates to a method of heating a 310S steel slab, which is a heat-resistant austenitic stainless steel, in a heating furnace for hot rolling, and more particularly, to a method of heating a 310S steel slab, Thereby improving the surface quality of the 310S steel hot-rolled coil and improving the water-rejection ratio.

Generally, a heating furnace for hot rolling a stainless steel slab is generally performed at a high temperature of 1200 ° C or higher. Examples of the heat source include coke oven gas (COG), blast furnace gas (BFG), propane gas (LPG) And liquefied natural gas (LNG). However, the carbon dioxide, water, and excess oxygen generated when these gases are burned act to accelerate the oxidation of the slab surface rapidly.

Typically, the amount of oxidation is governed by temperature, time, and composition of the combustion gas. In case of temperature, it is preferable to heat to the lowest possible temperature. However, in order to secure hot rolling property, it is necessary to heat before and after about 1250 ° C. The shorter the heating time is, the better the inner temperature of the slab should be. . On the other hand, the composition of the combustion gas varies according to the air: fuel ratio, so it is possible to set the conditions to minimize the oxidation amount regardless of the hot rolling property.

Korean Patent No. 93-18894 is a patent on a heating furnace for the austenitic stainless steel slab for hot rolling and Korean Patent No. 93-15397 for a patent on a heating furnace method of an Fe-Cr stainless steel slab . According to these patents, heating operation of 18% Cr-8% Ni steel, which is a representative grade of austenitic stainless steels, increases the excess oxygen in the combustion gas, thereby minimizing local oxidation and scale formation on the nodules formed in the base metal, In the case of 12% Cr steel and 17% Cr steel, which are the representative steels of Fe-Cr type stainless steels, oxides on the finger formed on the base metal are minimized by lowering the excess oxygen in the combustion gas It has been reported that not only the surface quality is improved but also the scale production amount is reduced by about 50%, thereby contributing to the improvement of the error rate.

The 310S steel is a steel containing 25% Cr-20% Ni and shows different oxidation behavior in the furnace compared to the previously reported steels. In other words, the reported steels form oxides in the furnace phase or finger phase due to the chromium film breakage in the furnace, while the 310S steel forms a stable chromium film due to the high chromium content, .

An object of the present invention is to provide a method of heating austenitic-type 310S stainless steel slab for heat resistance, which can improve the quality of hot-rolled surface by minimizing the amount of scale formation and improve the yield rate.

1 shows the heating curve of the slab surface layer for simulating the oxidation behavior of the heating furnace,

FIG. 2 is a graph showing the oxidation behavior of the 310S steel according to the combustion gas composition change,

FIG. 3 is a photograph showing the scale section EPMA analysis result of the sample oxidized for 210 minutes in the atmosphere of 4.7% ExO ₂ at 1250 ° C.,

FIG. 4 is a photograph showing a change in scale cross-sectional shape due to an excess oxygen change in the combustion gas.

In order to achieve the above object, the present invention provides a ferritic stainless steel comprising, by weight, at least 0.08% of C, 24-26% of Cr, 19-22% of Ni, 2% , The remainder Fe and other unavoidable impurities by heating the slab in a furnace by using a coke oven gas (COG) and a mixed gas of blast furnace gas or a similar composition for hot rolling the austenitic stainless steel slab of 310S Wherein the amount of excess oxygen in the heating furnace atmosphere is controlled to a volume ratio of 1.5% or less.

Hereinafter, the present invention will be described in more detail by way of examples.

The chemical composition of the Donny 310S steel used in the present invention is shown in Table 1 below.

FIG. 1 shows a heating curve of a surface layer of a slab for simulating a heating furnace oxidation behavior. In the initial stage of the heating furnace, the temperature is increased rapidly to about 600 ° C, but then it is increased at a rate of about 5-7 ° C / min and the surface layer reaches about 1250 ° C. It was then maintained at 1250 ° C for about 90 minutes for internal aging of the slabs.

Table 2 below shows the composition of the combustion gas by the air-fuel ratio. In the present invention, the amount of excess oxygen in the combustion gas is used as a measure of the composition of the combustion gas. Here, the excess oxygen amount (Excess Oxygen = Ex. ₀₂ ) refers to the amount of oxygen to be supplied in addition to the amount of oxygen required for complete combustion.

FIG. 2 shows the oxidation behavior of 310S steel by heating gas at a temperature of 1250.degree. The left side shows the weight increase of the specimen by oxidation and the right side shows the charging time in the heating furnace. As shown in FIG. 2, as the volume ratio of the excess oxygen in the combustion gas increases from 0.8% to 3.0%, the oxidation amount is increased to about 1.35 times. Here, [the volume ratio of the excess oxygen amount = the oxygen content in the combustion gas / the total combustion gas amount] means that the value is controlled to 1.5% or less, whereby the surge of oxidation amount can be suppressed.

This phenomenon is contrary to the oxidative behavior (Patent No. 93-18894) of the conventional austenitic stainless steel, which is caused by the difference in the Cr content. That is, when the Cr content is low, the activity of oxygen in the combustion atmosphere is increased to promote the formation of the chromium protective film to reduce the oxidation amount. On the other hand, when the Cr content is high, It is possible to minimize the thickness of the chrome protective coating by lowering the activity. In this case, it is expected that the improvement of the peelability of the scale and the reduction of the oxidation amount and the improvement of the surface quality due to the minimization of the internal oxide formation in the base metal can be expected.

FIG. 3 shows the EPMA mapping results of the scale section of the oxidized specimen at 1250 ° C -4.7% Ex.O ₂ atmosphere. The Fe-oxides formed on the outermost layer of the specimen after the oxidation of the furnace were peeled off during the cooling process, and only the shape of the oxide film formed on the surface of the base metal was shown in the photograph. As can be seen from the photograph, the scale formed on the surface of the base metal is Cr-oxide containing a part of Mn-oxide on the surface, and the internal oxide formed in the base metal is mainly composed of Si-oxide.

These oxides are known to cause surface defects such as slivers because they are difficult to remove in the stainless steel manufacturing process. Particularly, the thicker the film thickness, the larger the inner oxide size, or the deeper the penetration depth, the easier it is to remove. Therefore, to improve the surface quality of high-alloy stainless steels, it is very important to set the operating conditions that minimize the thickness of the protective coating and the formation of internal oxides.

4 shows changes in scale cross-sectional shape due to an increase in excess oxygen amount in the combustion gas. Referring to FIG. 4, as the excess oxygen increases, the thickness of the chromium film and the size and depth of the Si-oxide formed in the base metal are significantly increased.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, by reducing the excess oxygen in the atmosphere of the furnace during the hot rolling of the 310S steel slab, it is possible not only to improve the hot-rolled surface quality significantly, but also to improve the yield rate. Also, by lowering the excess oxygen (reducing the inflow amount of the air), the thermal efficiency of the fuel can be greatly improved.

Claims (1)

Austenite consisting of 0.08% or less of C, 24-26% of Cr, 19-22% of Ni, 2% or less of Mn, 1.5% or less of Si, 0.01% or less of B and the balance Fe and other unavoidable impurities A method for heating a slab using a coke oven gas (COG) or a blast furnace gas as a fuel to hot-roll a total 310S stainless steel slab, wherein the excess oxygen amount in the furnace atmosphere is controlled to be within a range of 1.5% Austenitic-type 310S stainless steel slab heating method.